87 300D - Only 8 Volts at Rear Stop Lamp - PeachParts Mercedes-Benz Forum

beaulieub · #1 06-15-2011, 12:09 PM

Passenger side, rear stop light only has 8 volts at the wire going into the connector. LIght doesn't work. Tried jiggling it thinking maybe a bad connection somewhere but can't find anything. All the other lights in the assembly (tail, backup, turn signal) work fine so its not a ground problem. The problem must be somewhere else in the harness and I'm not that good at searching for such things.
Might I be able to jumper a wire from the drivers side stop light connector across the trunk to the passenger side stop light? I realize the wire is probably not rated to carry twice the load but thought since it's only a stop light which is not used continually like a tail light is that it might be ok. Any comments or suggestions? Thanks in advance.

Jeremy5848 · #2 06-15-2011, 12:29 PM

The 1987 300D Turbo (sedan) has no connectors in the stop lamp circuit. It's a straight shot from the External Lamp Failure Monitoring Unit (N7) to the lamp fixture. So, just to cover all the bases, try a new lamp (or swap left-right) and make sure the base contacts are clean and solid. Check the plug that connects to the right lamp unit. It has six pins. Make sure all of the wires are solidly connected and not partially broken.

Then try a jumper from the left stop lamp, just to see if you can get the right one to work. I'm not sure what N7 will think of that. By the way, are you getting a lamp failure warning light on your dash? If a brake light isn't working, N7 should be saying something about it.

Finally, try removing N7 (it's the longest relay in the relay box, behind and part of the fuse box, you have to remove 6 screws to get the cover off) and cleaning all its contacts. Don't use anything rough, the silver plating on the pins will come off. Use a metal polish and q-tips. While you're in there, check the pins in the three sockets for N7. The right stop lamp connects to pin 3 of connector #2 (a 10-pin connector, the middle one in the row of three than connect to N7.

If you can't find anything wrong, go ahead and use the jumper. It won't hurt anything. N7 may not be happy but it won't smoke.

Jeremy

beaulieub · #3 06-15-2011, 03:03 PM

Jeremy, Thanks for the info and comments. Yes the light on the dash is lit telling me of a bulb problem. Already did the swapping of the bulbs and ruled out a bad bulb. That lead me to use the multi-meter and seeing only 8 volts on the stop lamp pin in the plug. All other pins have 12 volts but this one. Never thought about a relay problem. I will check back on that and report back. thanks again.

NH_Bert

beaulieub · #4 06-15-2011, 04:29 PM

Did the jumper wire and now I have 2 rear stop lights. N7 still doesn't like it and lights as soon as I press on the brake but I can live with that. Opened the relay box but didn't remove any of them just wiggled them a bit. It made no difference so I went ahead with the jumper wire. Thanks again for your help.

winmutt · #5 06-15-2011, 05:28 PM

Check resistance to ground? I found that the wires often get corroded at the ends underneah the sheeth.

Jeremy5848 · #6 06-15-2011, 08:01 PM

I forgot to ask -- your 8 volt measurement was made with someone's foot on the brake and you back at the stop lamp? That means that somewhere there's a wire that has either corroded, as winmutt says, or has mostly broken so that there are only one or two strands, not enough to carry the current needed to light the stop lamp.

The only alternative is an electronic failure in the bulb failure module, N7. You would have to get a spare (junkyards should have them or someone on the forum) and do a swap to see if it helps. The part number is 126 542 01 32 and they are common to all 124s and some 126s (S-class cars like the 300SD and 300SE, etc.). You don't want to know what the dealer would charge for a new one.